Hello, why a hash to be secure it cannot be reversed. When a pc sends a hashed output to another pc it normally includes the plain text as well so the other pc can compare the hash by hashing the clear text. What is wrong in being able to reverse the hash since the text is being sent in clear text as well ?

The text is not being sent in clear text. Usually the sender only sends the hash of the clear text, then the receiver calculates the same hash on the clear text it has in his possession, then both hashes are compared and if they match, this means that the sender knows the correct clear text. Feel free to substitute clear text with password